1. Field of the Invention
The present invention relates to high purity carbon fiber reinforced carbon composites (hereinafter referred to as C/C composites) having a reduced impurities content by using a high purity treatment. More particularly, the present invention relates to high-purity C/C composites useful as semiconductor material processing components, such as Czochralski (hereinafter referred to as CZ) furnace components and apparatus.
2. Description of the Background
FIG. 2 illustrates a pulling single crystal apparatus used in the CZ process for manufacturing a single crystal ingot for use as a material of a semiconductor wafer, for example. As shown in FIG. 2, the CZ apparatus is so structured that a raw material in a quartz crucible 1 is heated to a high temperature by a heater 2 disposed around the quartz crucible 1 so that the raw material can be converted into the melt 3 which is pulled under vacuum pressure to form the single crystal ingot 4.
The structural elements, such as a crucible 5 supporting the quartz crucible 1 and an upper ring 6, an inner shield 7 and others which are subjected to radiant heat of the heater 2, are exposed to high temperature when pulling a single crystal ingot 4 from the quartz crucible 1 within a molten silicon.
Accordingly, the structural elements must be formed of a material that can maintain a prescribed mechanical strength under high temperature. Further, the structural elements must be formed of a material of high-purity, because impurities, such as metals, contained in the structural elements lead to crystal defects in orientation of the solidifying single crystal ingot 4 and also lead to reduced purity, when leaked during manufacturing. In general, a high purity graphite having excellent mechanical properties at high temperature and having high-purity is used for the structural elements of the CZ apparatus (Japanese Patent Publication No. Hei 6(1994)-35325). Recently, with increasing diameter of the single crystal, the single crystal pulling apparatus used in the CZ process has also been increased in size. This leads to difficult handling because of the increased weight of the graphite elements and a problem in reducing the effective processing size of the interior of the apparatus.
The C/C composites have the properties of lightweight and excellent mechanical strength, as compared with the graphite material. Thus, even when reducing thickness, the structural elements of the C/C composites exhibit a strength equal to that of the graphite material, allowing effective use of the processing chamber of the apparatus. In addition, by being lightweight, the apparatus may be easily handled. With such advantages, crucible components used in the CZ apparatus having a large diameter are now moving from those made of graphite to those made of C/C composites.
Recently, elements for use in a CZ apparatus made of high purity C/C composites have been proposed. For example, U.S. Pat. Nos. 5,683,281 and 5,800,924 disclose high purity C/C composites subjected to degasification at 2,400xc2x0 C. to 3,000xc2x0 C. under an inert gas atmosphere in the carbon fiber stage; impregnation with high purity pitch or resin for densification; graphitization; and high purification at 2,400xc2x0 C. to 3,000xc2x0 C. under a halogen gas atmosphere, so that the impurities of Ag, Al, Ba, Be, Ca, Cd, Co, Cr, Cu, K, Mg, Mn, Mo, Na, Ni, P, Pb, Sr and Zn can be reduced below the detection limit by inductively coupled plasma spectroscopy (ICP). U.S. Pat. No. 5,616,175 discloses 3-D C/C composites for a pulling single crystal apparatus which is impregnated with the carbon material which results in a matrix which is then graphitized, followed by high purification under a halogen gas, so that the impurities are not more than 10 ppm and whose surface is coated with pyrolytic carbon or silicon carbide, for preventing reaction with the gas generated from the melt when the single crystal is pulled.
In the single crystal solidification of the pulling silicon single crystal in the CZ process, well known impurities, such as V and Fe, cause crystal defects in orientation of the solidifying of the single crystal. Therefore, the total content of impurities must be reduced, particularly by decreasing the V and Fe impurities for using the CZ process. In U.S. Pat. Nos. 5,683,281 and 5,800,924, the impurities of V and Fe were detected by ICP. The amounts of the impurities of V and Fe are still too great to control the crystal defects in orientation of the solidifying single crystal.
It is an object of the present invention to provide high purity C/C composites with reduced impurities content, such as V and Fe, by high purity treatment.
The high purity C/C composites are formed by graphitizing a molded member packed with carbon fibers and carbon material of a matrix. The carbon fibers are highly purified under halogen gas atmosphere. The purified carbon fibers are molded into the desired shape on a tool or in a die and impregnating the carbon material of the matrix. The molded member packed with carbon fibers and carbon material of the matrix are either independently or simultaneously graphitized with high-purification under halogen gas atmosphere, and then are highly purified under a vacuum/halogen gas atmosphere in order to highly purify the C/C composites. According to the present process, impurities are greatly reduced. The impurities content of V and Fe, for example, are not more than 0.12 ppm and 0.10 ppm by ICP optical emission analysis, respectively.
The carbon material of the matrix impregnated carbon fibers become graphite fibers. The carbon material of the matrix around the graphite fibers also becomes graphite and coats the graphite fibers. The present high-purity C/C composites contain two graphites namely, the high-purity graphite fibers and the high-purity graphite matrix. According to the structure of the C/C composites, the impurities may be difficult to dissolve from the inside of the graphite fibers.
The C/C composites of U.S. Pat. Nos. 5,683,281, 5,800,924 and 5,616,175 are highly-purified under halogen gas at one time after graphitizing the molded C/C composites which are structured to coat the graphite matrix on the graphite fibers. Accordingly by structure, the C/C composites may be mainly purified around the outside surface of the graphite matrix, and the impurities of the inside of the graphite matrix and of the graphite fibers may be difficult to purify in the purification process thereof. The graphite fibers of the present invention are highly purified because the fibers are purified under halogen gas atmosphere before molding and graphitizing, and then the graphitized and molded C/C composites are either independently or simultaneously graphitized with high-purification under halogen gas atmosphere. The purity of the resulting C/C composites is not more than 30 ppm of ash content.